Phillip M Reyes

St. Joseph's Hospital and Medical Center (AZ, USA), Phoenix, Arizona, United States

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Publications (31)75.6 Total impact


  • No preview · Article · Jan 2016 · Turkish neurosurgery
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    ABSTRACT: OBJECT The effects of obesity on lumbar biomechanics are not fully understood. The aims of this study were to analyze the biomechanical differences between cadaveric L4-5 lumbar spine segments from a large group of nonobese (body mass index [BMI] < 30 kg/m(2)) and obese (BMI ≥ 30 kg/m(2)) donors and to determine if there were any radiological differences between spines from nonobese and obese donors using MR imaging. METHODS A total of 168 intact L4-5 spinal segments (87 males and 81 females) were tested using pure-moment loading, simulating flexion-extension, lateral bending, and axial rotation. Axial compression tests were performed on 38 of the specimens. Sex, age, and BMI were analyzed with biomechanical parameters using 1-way ANOVA, Pearson correlation, and multiple regression analyses. MR images were obtained in 12 specimens (8 from obese and 4 from nonobese donors) using a 3-T MR scanner. RESULTS The segments from the obese male group allowed significantly greater range of motion (ROM) than those from the nonobese male group during axial rotation (p = 0.018), while there was no difference between segments from obese and nonobese females (p = 0.687). There were no differences in ROM between spines from obese and nonobese donors during flexion-extension or lateral bending for either sex. In the nonobese population, the ROM during axial rotation was significantly greater for females than for males (p = 0.009). There was no significant difference between sexes in the obese population (p = 0.892). Axial compressive stiffness was significantly greater for the obese than the nonobese population for both the female-only group and the entire study group (p < 0.01); however, the difference was nonsignificant in the male population (p = 0.304). Correlation analysis confirmed a significant negative correlation between BMI and resistance to deformation during axial compression in the female group (R = -0.65, p = 0.004), with no relationship in the male group (R = 0.03, p = 0.9). There was also a significant negative correlation between ROM during flexion-extension and BMI for the female group (R = -0.38, p = 0.001), with no relationship for the male group (R = 0.06, p = 0.58). Qualitative analysis using MR imaging indicated greater facet degeneration and a greater incidence of disc herniations in the obese group than in the control group. CONCLUSIONS Based on flexibility and compression tests, lumbar spinal segments from obese versus nonobese donors seem to behave differently, biomechanically, during axial rotation and compression. The differences are more pronounced in women. MR imaging suggests that these differences may be due to greater facet degeneration and an increased amount of disc herniation in the spines from obese individuals.
    No preview · Article · Dec 2015 · Journal of Neurosurgery Spine
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    ABSTRACT: Background: The use of pedicle screws is the gold standard for supplemental posterior fixation in lateral interbody fusion. Information about the performance of transfacet pedicle screws compared to standard pedicle screws and rods in the upper lumbar spine with or without a lateral interbody fusion device in place is limited. Methods: Fifteen fresh frozen human cadaveric lumbar spine segments (T12-L4) were studied using standard pure moment flexibility tests. Specimens were divided into two groups to receive either bilateral transfacet pedicle screws (n=8) or bilateral pedicle screws (n=14). Stability of each motion segment (L1-L2 and L2-L3) was evaluated intact, with posterior instrumentation with an intact disc, with posterior instrumentation and a lateral interbody fusion device in place, and following cyclic loading with the interbody device and posterior instrumentation still in place. Both raw values of motion (range of motion, lax zone and stiff zone) and normalized mobility (ratios to intact) were analyzed for each case. Findings: In terms of immediate stability, transfacet pedicle screws performed equivalent to similarly sized pedicle screws, both with intact disc and with lateral interbody fusion device in all directions of loading. Stability following cyclic loading decreased significantly during lateral bending and axial rotation. Interpretation: Posterior fixation with transfacet pedicle screws provides equivalent immediate stability to similarly sized pedicle screws. However, in the presence of a lateral interbody fusion device, pedicle screws seem to resist loosening more and may be a better option for fusion in the upper lumbar spine.
    No preview · Article · Oct 2015 · Clinical biomechanics (Bristol, Avon)
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    ABSTRACT: OBJECT The Universal Clamp Spinal Fixation System (UC) is a novel sublaminar connection for the spine that is currently used in conjunction with pedicle screws at the thoracic levels for the correction of scoliosis. This device allows the surgeon to attach rods and incorporate a pedicle screw construction. The flexible composition of the UC should provide flexibility intermediate to the uninstrumented spine and an all-screw construct. This hypothesis was tested in vitro using nondestructive flexibility testing of human cadaveric spine segments. METHODS Six unembalmed human cadaveric thoracic spine segments from T-3 to T-11 were used. The specimens were tested under the following conditions: 1) intact; 2) after bilateral screws were placed at T4-T10 and interconnected with longitudinal rods; 3) after placement of a hybrid construction with screws at T-4, T-7, and T-10 with an interconnecting rod on one side and screws at T-4 and T-10 with the UC at T5-9 on the contralateral side; (4) after bilateral screws were placed at T-4 and T-10 and interconnected with rods and bilateral UC were placed at T5-9; and 5) after bilateral screws at T-4 and T-10 were placed and interconnected with rods and bilateral sublaminar cables were placed at T5-9. Pure moments of 6.0 Nm were applied while optoelectronically recording 3D angular motion. RESULTS Bilateral UC placement and bilateral sublaminar cables both resulted in a significantly greater range of motion than bilateral pedicle screws during lateral bending and axial rotation, but not during flexion or extension. There were no differences in stability between bilateral UC and bilateral cables. The construct with limited screws on one side and UC contralaterally showed comparable stability to bilateral UC and bilateral cables. CONCLUSIONS These results support using the UC as a therapeutic option for spinal stabilization because it allows comparable stability to the sublaminar cables and provides flexibility intermediate to that of the uninstrumented spine and an all-screw construct. Equivalent stability of the hybrid, bilateral UC, and bilateral cable constructs indicates that 6-level UC provides stability comparable to that of a limited (3-point) pedicle screw-rod construct.
    No preview · Article · Oct 2015 · Journal of neurosurgery. Spine
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    ABSTRACT: Background: Arthrodesis is optimized when the structural graft occupies most of the surface area within a disc space. The transforaminal corridor inherently limits interbody size. Objective: To evaluate the biomechanical implications of nested interbody spacers (ie, a second curved cage placed behind a first) to increase disc space coverage in transforaminal approaches. Methods: Seven lumbar human cadaveric specimens (L3-S1) underwent nondestructive flexibility and axial compression testing intact and after transforaminal instrumentation at L4-L5. Specimens were tested in 5 conditions: (1) intact, (2) interbody, (3) interbody plus bilateral pedicle screws and rods (PSR), (4) 2 nested interbodies, and (5) nested interbodies plus PSR. Results: Mean range of motion (ROM) with 1 interbody vs 2 nested interbodies, respectively, was: flexion, 101% vs 85%; extension, 97% vs 92%; lateral bending, 127% vs 132%; and axial rotation, 145% vs 154%. One interbody and nested interbodies did not differ significantly by loading mode (P > .10). With PSR, ROM decreased significantly compared with intact, but not between interbody and interbody plus PSR or 2 interbodies plus PSR (P > .80). Mean vertical height during compressive loading (ie, axial compressive stiffness) was significantly different with nested interbodies vs 1 interbody alone (P < .001) (compressive stiffness, 89% of intact vs 67% of intact, respectively). Conclusion: Inserting a second interbody transforaminally is anatomically feasible and nearly doubles the disc space covered without affecting ROM. Compressive stiffness significantly increased with nested interbodies, and foraminal height increased. Evaluation of the clinical safety and efficacy of nested interbodies is underway. Abbreviations: BMD, bone mineral densityPSR, pedicle screws and rodsROM, range of motion.
    Full-text · Article · Sep 2015 · Neurosurgery
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    ABSTRACT: Non-destructive flexibility tests were performed in vitro, comparing multiple conditions of fixation in a single group of specimens. To compare the biomechanical behavior of the lumbar spine in the intact condition, after implanting a novel motion stabilizer, and after implanting a rigid fixator. Two specific scenarios that may benefit from dynamic lumbar stabilization are single-level moderate instability, where the stabilizing tissues are relatively incompetent, and juxta-level to fusion, where the last instrumented level requires intermediate stiffness ("topping off") to prevent transfer of high stresses from the stiffer fusion construct to the intact adjacent levels. Both scenarios were evaluated in vitro. Seven human cadaveric L2-S1 segments were tested (1) intact, (2) after moderate destabilization, (3) after 2-level hybrid posterior fixation, consisting of bilateral dynamic pedicle screws at L4 interconnected with rigid rods to standard pedicle screws at L5 and S1, (4) after 2-level rigid fixation, (5) after 1-level (L4-L5) dynamic fixation, and (6) after 1-level rigid fixation. In each condition, angular range of motion (ROM) and sagittal instantaneous axis of rotation (IAR) were assessed. In 1-level constructs, dynamic hardware allowed 104% of intact ROM whereas rigid hardware allowed 49% of intact ROM. Relative to the intact, the IAR was shifted significantly farther posterior by rigid 1-level instrumentation than dynamic 1-level instrumentation. In 2-level constructs, the dynamic level allowed significantly greater ROM than the rigid level in all directions but allowed significantly less ROM than the intact level in all directions except axial rotation. Dynamic instrumentation shifted the IAR less than rigid instrumentation, providing more favorable kinematics. This dynamic stabilizer provided 1-level ROM that was close to intact ROM during all loading modes in vitro. In the "topping off" construct, the dynamic segment allowed intermediate ROM to give balanced transitional flexibility. N/A.
    No preview · Article · Sep 2015 · Spine
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    ABSTRACT: Study Design. A human cadaveric biomechanical study of two sacroiliac joint fusion implant placement techniques.Objective. To evaluate and compare the biomechanical properties of two implant placement techniques for sacroiliac joint fusion.Summary of Background Data. Minimally invasive placement of sacroiliac joint fusion implants is a potential treatment for sacroiliac joint disruptions and degenerative sacroiliitis. Biomechanical studies of screw fixation within the sacrum have shown that placement and trajectory are important in the overall stability of the implant. Although clinical results have been promising, there is the possibility that a more optimal arrangement of implants may exist.Methods. Bilateral sacroiliac joints in seven cadaveric lumbopelvic (L4-pelvis) specimens were tested using a single leg stance model. All joints were tested intact, pubic symphysis sectioned, and treated (three sacroiliac joint fusion implants). The implants were laterally placed using either a posterior or a trans-articular placement technique. The posterior technique places the implants inline in the inlet view, parallel in the outlet view, and parallel to the posterior sacral body in the lateral view. The trans-articular technique places all implants across the articular portion of the sacroiliac joint. For all conditions tested, the range of motion was tested in flexion-extension, lateral bending, and axial rotation.Results. The posterior technique significantly reduced the range of motion in flexion-extension, lateral bending, and axial rotation by 27% ± 24% (p = .024), 28% ± 26% (p = .028), and 32% ± 21% (p = .008), respectively. The trans-articular technique significantly reduced the range of motion in flexion-extension, lateral bending, and axial rotation by 41% ± 31% (p = .013), 36% ± 38% (p = .049), and 36% ± 28% (p = .015), respectively. No significant differences were detected between the posterior and trans-articular placement techniques (p>.25).Conclusions. Posterior and trans-articular placement of sacroiliac joint fusion implants stabilized the sacroiliac joint in flexion-extension, lateral bending, and axial rotation.
    No preview · Article · Feb 2015 · Spine
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    ABSTRACT: Introduction Sacroiliac (SI) joint pain has become a recognized factor in low back pain. The purpose of this study was to investigate the effect of a minimally invasive surgical SI joint fusion procedure on the in vitro biomechanics of the SI joint before and after cyclic loading. Methods Seven cadaveric specimens were tested under the following conditions: intact, posterior ligaments (PL) and pubic symphysis (PS) cut, treated (three implants placed), and after 5,000 cycles of flexion–extension. The range of motion (ROM) in flexion–extension, lateral bending, and axial rotation was determined with an applied 7.5 N · m moment using an optoelectronic system. Results for each ROM were compared using a repeated measures analysis of variance (ANOVA) with a Holm–Šidák post-hoc test. Results Placement of three fusion devices decreased the flexion–extension ROM. Lateral bending and axial rotation were not significantly altered. All PL/PS cut and post-cyclic ROMs were larger than in the intact condition. The 5,000 cycles of flexion–extension did not lead to a significant increase in any ROMs. Discussion In the current model, placement of three 7.0 mm iFuse Implants significantly decreased the flexion–extension ROM. Joint ROM was not increased by 5,000 flexion–extension cycles.
    Full-text · Article · May 2014 · Medical Devices: Evidence and Research

  • No preview · Article · May 2014 · Global Spine Journal
  • Marco T Reis · Phillip M Reyes · Neil R Crawford
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    ABSTRACT: A new anchored cervical interbody PEEK spacer was devised that uses only 2 integrated variable angle screws diagonally into the adjacent vertebral bodies instead of the established device that uses 4 diagonal fixed-angle screws. To compare in vitro the stability provided by the new 2-screw interbody spacer to that of the 4-screw spacer and a 4-screw anterior plate plus independent PEEK spacer. Three groups of cadaveric specimens were tested with 2-screw anchored cage (N=8), 4-screw anchored cage (N=8), and standard plate/cage (N=16). Pure moments (1.5Nm) were applied to induce flexion, extension, lateral bending, and axial rotation while measuring 3-D motion optoelectronically. In all three groups, the mean range of motion (ROM) and lax zone (LZ) were significantly reduced relative to intact after discectomy and fixation. The 2-screw anchored cage allowed significantly greater ROM (p<0.05) than the standard plate during flexion, extension, and axial rotation and allowed significantly greater ROM than the 4-screw cage during extension and axial rotation. The 4-screw anchored cage did not allow significantly different ROM or LZ than the standard plate during any loading mode. The 2-screw variable angle anchored cage significantly reduces ROM relative to intact. Greater stability can be achieved, especially during extension and axial rotation, by using the 4-screw cage or standard plate plus cage.
    No preview · Article · Mar 2014 · Neurosurgery
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    ABSTRACT: The transforaminal lumbar interbody fusion (TLIF) technique supplements posterior instrumented lumbar spine fusion and has been tested with different types of screw fixation for stabilization. Transforaminal lumbar interbody fusion is usually placed through a unilateral foraminal approach after unilateral facetectomy, although extraforaminal entry allows the facet to be spared. To characterize the biomechanics of L4-L5 lumbar motion segments instrumented with bilateral transfacet pedicle screw (TFPS) fixation versus bilateral pedicle screw-rod (PSR) fixation in the setting of intact facets and native disc or after discectomy and extraforaminal placement of a TLIF technology graft. Human cadaveric lumbar spine segments were biomechanically tested in vitro to provide a nonpaired comparison of four configurations of posterior and interbody instrumentation. Fourteen human cadaveric spine specimens (T12-S1) underwent standard pure moment flexibility tests with intact L4-L5 disc and facets. Seven were studied with intact discs, after TFPS fixation, and then with TLIF and TFPS fixation. The others were studied with intact discs, after PSR fixation, and then combined with extraforaminally placed TLIF. Loads were applied about anatomic axes to induce flexion-extension, lateral bending, and axial rotation. Three-dimensional specimen motion in response to applied loads during flexibility tests was determined. A nonpaired comparison of the four configurations of posterior and interbody instrumentation was made. Transfacet pedicle screw and PSR, with or without TLIF, significantly reduced range of motion during all directions of loading. Transfacet pedicle screw provided greater stability than PSR in all directions of motion except lateral bending. In flexion, TFPS was more stable than PSR (p=.042). A TLIF device provided less stability than the intact disc in constructs with TFPS and PSR. These results suggest that fixation at L4-L5 with TFPS is a promising alternative to PSR, with or without TLIF. A TLIF device was less stable than the native disc with both methods of instrumentation presumably because of a fulcrum effect from a relatively small footplate. Additional interbody support may be considered for improved biomechanics with TLIF.
    No preview · Article · Nov 2013 · The spine journal: official journal of the North American Spine Society
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    ABSTRACT: In vitro nondestructive flexibility testing of the CerviCore total disc replacement (TDR) was performed. It was hypothesized that TDR would not significantly alter biomechanics relative to intact, whereas rigid fixation would cause significant changes. To assess the ability of a cervical metal-on-metal saddle-shaped TDR to replicate normal biomechanics in vitro. Human cadaveric flexibility experiment. Nine human cadaveric C3-T1 specimens were tested intact, after TDR and after anterior plating. Flexion, extension, lateral bending, and axial rotation were induced by pure moments; flexion-extension was then repeated using a simplified muscle force model with 70-N follower load. Optical markers measured three-dimensional intervertebral motion, and eight points of laminar surface strain were recorded near the left and right C5-C6 facet joints. Biomechanical parameters studied included range of motion (ROM), lax zone (LZ), angular coupling pattern, sagittal instantaneous axis of rotation (IAR), and facet loads normal to the facet joint plane. Mean values of parameters were compared statistically using repeated measures analysis of variance and Holm-Sidak tests. Total disc replacement caused significant reduction in ROM during extension (p=.004) and significant reduction in LZ during lateral bending (p=.01). However, plating significantly reduced both ROM and LZ during flexion, extension, and lateral bending (p<.006). Sagittal IAR shifted relative to intact by 3.6 mm after TDR (p>.05) and 6.5 mm after plating (p>.05). Coupled axial rotation/degree lateral bending was 99% of intact after TDR but 76% of intact after plating (p=.15). Coupled lateral bending/degree axial rotation was 95% of intact after TDR but 85% of intact after plating (p=.43). Neither construct altered facet loads from intact. With regard to ROM, LZ, IAR, and coupling, deviations from intact biomechanics were less substantial after TDR than after plating. Facet load alterations were minimal with either construct. Our results show that this particular TDR permits ROM and maintains some measures of kinematics in a cadaver model.
    Full-text · Article · Oct 2013 · The spine journal: official journal of the North American Spine Society
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    ABSTRACT: BACKGROUND:: Transitioning from rigid to flexible hardware at the distal rostral or caudal lumbar or lumbosacral level hypothetically maintains motion at the transition level and protects the transition level and intact adjacent levels from stresses caused by fusion. OBJECTIVE:: To biomechanically compare transitional and rigid constructs to uninstrumented specimens in vitro. METHODS:: Human cadaveric L2-S1 segments were tested (1) intact, (2) after L5-S1 rigid pedicle screw-rod fixation, (3) after L4-S1 rigid pedicle screw-rod fixation, and (4) after hybrid fixation rigidly spanning L5-S1 and dynamically spanning L4-L5. Pure moments (maximum 7.5 Nm) induced flexion, extension, lateral bending, and axial rotation while motion was recorded optoelectronically. Additionally, specimens were studied in flexion/extension with a 400-N compressive follower load. Strain gauges on laminae were used to extract facet loads. RESULTS:: The range of motion (ROM) at the transition segment (L4-L5) for the hybrid construct was significantly less than for the intact condition and significantly greater than for the rigid 2-level construct during lateral bending and axial rotation but not during flexion or extension. Sagittal axis of rotation at L4-L5 shifted significantly after rigid 2-level or hybrid fixation (p<0.003), but shifted significantly farther posterior and rostral with rigid fixation (p<0.02). Instrumentation altered L4-L5 facet load at more than L3-L4 facet load. CONCLUSION:: The effect of the dynamic rod segment on the kinematics of the transition level was less pronounced than that of a fully rigid construct in vitro using this particular rod system. This experimental model detected no biomechanical alterations at adjacent intact levels with hybrid or rigid systems.
    Full-text · Article · Jun 2013 · Neurosurgery
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    ABSTRACT: STUDY DESIGN:: A new experimental protocol was applied utilizing a simplified postural control model. Multiple constructs were tested nondestructively by interconnecting segmental rods to screws. OBJECTIVE:: To investigate how posture and distribution of segmental angles under physiologic loads are affected by combined cervical arthroplasty and fusion. SUMMARY OF BACKGROUND DATA:: Previous studies of biomechanics of multilevel arthroplasty have focused on range of motion and intradiscal pressure. No previous study has investigated postural changes and segmental angle distribution. METHODS:: In seven human cadaveric C3-T1 specimens, C4-C5, C5-C6 and C6-C7 discs were replaced with ProDisc-C (Synthes). Combinations of fusion ("f") adjacent to arthroplasty ("A") were simulated at C4-C5, C5-C6 and C6-C7 respectively: fAA, AfA, AAf, ffA, fAf, Aff, fff. C3-C4 and C7-T1 remained intact. A compressive belt apparatus simulated normal muscle co-contraction and gravitational preload while C3-C4, C4-C5, C5-C6, C6-C7 and C7-T1 motions were tracked independently. Parameters studied were segmental postural compensation, neutral buckling, and shift in sagittal plane instantaneous axis of rotation (IAR). RESULTS:: With one or more levels unfused, the arthroplasty levels preferentially moved toward upright posture before the intact levels. Neutral buckling was greatest for 3-level arthroplasty, less for 2-level arthroplasty, and least for 1-level arthroplasty. Among the three 1-level arthroplasty groups (ffA, fAf, Aff), arthroplasty at the caudalmost level resulted in significantly greater buckling than with arthroplasty rostralmost or at mid-segment (P<0.04, ANOVA/Holm-Sidak). Although IAR location was related to buckling, this correlation did not reach significance (P=0.112). CONCLUSION:: Arthroplasty levels provide the "path of least resistance," through which the initial motion is more likely to occur. The tendency for specimens to buckle under vertical compression became greater with more arthroplasty levels. Buckling appeared more severe with arthroplasty more caudal. Buckling only moderately correlated to shifts in IAR, meaning slight malpositioning of the devices would not necessarily cause buckling.
    No preview · Article · May 2013 · Journal of spinal disorders & techniques
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    ABSTRACT: BACKGROUND CONTEXT: Novel dual-threaded screws are configured with overlapping (doubled) threads only in the proximal shaft to improve proximal cortical fixation. PURPOSE: Tests were run to determine whether dual-threaded pedicle screws improve pullout resistance and increase fatigue endurance compared with standard pedicle screws. STUDY DESIGN/SETTING: In vitro strength and fatigue tests were performed in human cadaveric vertebrae and in polyurethane foam test blocks. PATIENT SAMPLE: Seventeen cadaveric lumbar vertebrae (14 pedicles) and 40 test sites in foam blocks were tested. OUTCOME MEASURES: Measures for comparison between standard and dual-threaded screws were bone mineral density (BMD), screw insertion torque, ultimate pullout force, peak load at cyclic failure, and pedicular side of first cyclic failure. METHODS: For each vertebral sample, dual-threaded screws were inserted in one pedicle and single-threaded screws were inserted in the opposite pedicle while recording insertion torque. In seven vertebrae, axial pullout tests were performed. In 10 vertebrae, orthogonal loads were cycled at increasing peak values until toggle exceeded threshold for failure. Insertion torque and pullout force were also recorded for screws placed in foam blocks representing healthy or osteoporotic bone porosity. RESULTS: In bone, screw insertion torque was 183% greater with dual-threaded than with standard screws (p<.001). Standard screws pulled out at 93% of the force required to pull out dual-threaded screws (p=.42). Of 10 screws, five reached toggle failure first on the standard screw side, two screws failed first on the dual-threaded side, and three screws failed on both sides during the same round of cycling. In the high-porosity foam, screw insertion torque was 60% greater with the dual-threaded screw than with the standard screw (p=.005), but 14% less with the low-porosity foam (p=.07). Pullout force was 19% less with the dual-threaded screw than with the standard screw in the high-porosity foam (p=.115), but 6% greater with the dual-threaded screw in the low-porosity foam (p=.156). CONCLUSIONS: Although dual-threaded screws required higher insertion torque than standard screws in bone and low density foam, dual-threaded and standard pedicle screws exhibited equivalent axial pullout and cyclic fatigue endurance. Unlike single-threaded screws, the mechanical performance of dual-threaded screws in bone was relatively independent of BMD. In foam, the mechanical performance of both types of screws was highly dependent on porosity.
    Full-text · Article · Apr 2013 · The spine journal: official journal of the North American Spine Society
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    ABSTRACT: Object: Endoscopic endonasal approaches to the craniovertebral junction (CVJ) and clivus are increasingly performed for ventral skull-base pathology, but the biomechanical implications of these approaches have not been studied. The aim of this study was to investigate the spinal biomechanics of the CVJ after an inferior-third clivectomy and anterior intradural exposure of the foramen magnum as would be performed in an endonasal endoscopic surgical strategy. Methods: Seven upper-cervical human cadaveric specimens (occiput [Oc]-C2) underwent nondestructive biomechanical flexibility testing during flexion-extension, axial rotation, and lateral bending at Oc-C1 and C1-2. Each specimen was tested intact, after an inferior-third clivectomy, and after ligamentous complex dissection simulating a wide intradural exposure using an anterior approach. Angular range of motion (ROM), lax zone, and stiff zone were determined and compared with the intact state. Results: Modest, but statistically significant, hypermobility was observed after inferior-third clivectomy and intradural exposure during flexion-extension and axial rotation at Oc-C1. Angular ROM increased incrementally between 6% and 12% in flexion-extension and axial rotation. These increases were primarily the result of changes in the lax zone. No significant changes were noted at C1-2. Conclusions: Inferior-third clivectomy and an intradural exposure to the ventral CVJ and foramen magnum resulted in hypermobility at Oc-C1 during flexion-extension and axial rotation. Although the results were statistically significant, the modest degree of hypermobility observed compared with other well-characterized CVJ injuries suggests that occipitocervical stabilization may be unnecessary for most patients.
    No preview · Article · Feb 2013 · Journal of neurosurgery. Spine
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    ABSTRACT: Study Design: Seven different combinations of posterior screw fixation, with or without interbody support, were compared in vitro using nondestructive flexibility tests.Objective: To study the biomechanical behavior of a new cortical screw (CS) fixation construct relative to the traditional pedicle screw (PS) construct.Summary of Background Data: The CS is an alternative to the PS for posterior fixation of the lumbar spine. The CS trajectory is more sagittally and cranially oriented than the PS, being anchored in the pars interarticularis. Like PS fixation, CS fixation uses interconnecting rods fastened with top-locking connectors. Stability after bilateral CS fixation was compared to stability after bilateral PS fixation in the setting of intact disc and with direct lateral interbody fixation (DLIF) or transforaminal lateral interbody fixation (TLIF) support.Methods: Standard nondestructive flexibility tests were performed in cadaveric lumbar specimens, allowing unpaired comparisons of specific conditions from 28 specimens (4 groups of 7) within a larger experiment of multiple hardware configurations. Conditions tested and group from which results originated were (1) intact [all groups], (2) with L3-L4 bilateral PS/rods [Group 1], (3) with bilateral CS/rods [Group 2], (4) with DLIF [Group 3], (5) with DLIF+CS/rods [Group 4], (6) with DLIF+PS/rods [Group 3], (7) with TLIF+CS/rods [Group 2], and (8) with TLIF+PS/rods [Group 2]. To assess spinal stability, the mean range of motion (ROM), lax zone (LZ), and stiff zone (SZ) at L3-L4 were compared during flexion-extension, lateral bending, and axial rotation.Results: With intact disc, stability was equivalent after PS/rod and CS/rod fixation, except that PS/rod fixation was stiffer during axial rotation. With DLIF support, there was no significant difference in stability between PS/rod and CS/rod fixation. With TLIF support, PS/rod fixation was stiffer than CS/rod fixation during lateral bending.Conclusion: Bilateral CS/rod fixation provided about the same stability in cadaveric specimens as PS/rod fixation regardless of the presence of interbody, TLIF, or DLIF support.
    No preview · Article · Oct 2012 · Spine
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    ABSTRACT: The Food and Drug Administration has not cleared the following medical devices for the use described in this study. The following medical devices are being discussed for an off-label use: cervical lateral mass screws. As an alternative for cases in which the anatomy and spatial relationship between C-2 and a vertebral artery precludes insertion of C-2 pedicle/pars or C1-2 transarticular screws, a technique that includes opposing laminar hooks (claw) at C-2 combined with C-1 lateral mass screws may be used. The biomechanical stability of this alternate technique was compared with that of a standard screw-rod technique in vitro. Flexibility tests were performed in 7 specimens (occiput to C-3) in the following 6 different conditions: 1) intact; 2) after creating instability and attaching a posterior cable/graft at C1-2; 3) after removing the graft and attaching a construct comprising C-1 lateral mass screws and C-2 laminar claws; 4) after reattaching the posterior cable-graft at C1-2 (posterior hardware still in place); 5) after removing the posterior cable-graft and laminar hooks and placing C-2 pedicle screws interconnected to C-1 lateral mass screws via rod; and 6) after reattaching the posterior cable-graft at C1-2 (screw-rod construct still in place). All types of stabilization significantly reduced the range of motion, lax zone, and stiff zone compared with the intact condition. There was no significant biomechanical difference in terms of range of motion or lax zone between the screw-rod construct and the screw-claw-rod construct in any direction of loading. The screw-claw-rod technique restricts motion much like the standard Harms technique, making it an acceptable alternative technique when aberrant arterial anatomy precludes the placement of C-2 pars/pedicle screws or C1-2 transarticular screws.
    No preview · Article · Jul 2012 · Journal of neurosurgery. Spine
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    ABSTRACT: In vitro assessment of rib cage biomechanics in the region of true ribs with the ribs intact then sequentially resected in 5 steps. To determine the contribution of the rib cage to thoracic spine stability and kinematics. Previous in vitro studies of rib cage biomechanics have used animal spines or human cadaveric spines with ribs left unsecured, limiting the ability of the ribs to contribute to stability. Eight upper thoracic specimens that included 4 ribs and sternum were tested in special fixtures that disallowed relative movement of the distal ribs and their vertebrae. While applying 7.5 Nm pure moments in 3 planes, angular motion at the middle motion segment was studied in intact specimens and then (1) after splitting the sternum, (2) after removing the sternum, (3) after removing 50% of ribs, (4) after removing 75% of ribs, and (5) after disarticulating and completely removing ribs. During flexion/extension, the sternum and anterior rib cage most contributed to stability. During lateral bending, the posterior rib cage most contributed to stability. During axial rotation, stability was directly related to the proportion of ribs remaining intact. On average, intact ribs accounted for 78% of thoracic stability. An intact rib cage shifted the axis of rotation unpredictably, but its position remained consistent after partial resection of the ribs. During lateral bending, coupled axial rotation was mild and unaffected by ribs. Because of testing methodology, the rib cage accounted for a greater percentage of thoracic stability than previously estimated. Different rib cage structures resisted motion in different loading planes.
    Full-text · Article · Dec 2011 · Spine
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    ABSTRACT: Most one-level anterior cervical plates use two screws per vertebra (four screws in total). No study has addressed whether a simplified plate using one screw per vertebra is adequate for one-level fixation. To compare stability achieved by four-screw and two-screw plates after discectomy and placement of interbody spacer. Nondestructive multidirectional flexibility tests were performed in three independent groups of cadaveric spines to assess spinal stability after instrumentation. Human cadaveric C4-C7 specimens were tested intact and after discectomy followed by placement of a polyetheretherketone interbody graft and an anterior plate. Rigid two-screw (n=8), semiconstrained four-screw (n=8), and rigid four-screw (n=8) plates were compared. Nonconstraining pure moments were applied under load control (maximum 1.5 Nm) to induce flexion, extension, lateral bending, and axial rotation, whereas vertebral motion was measured optoelectronically. Mean range of motion (ROM) was compared among groups. All three plates significantly reduced ROM relative to normal in all directions of loading (p<.003). Mean ROMs±standard deviation (and corresponding intergroup p value) for rigid two-screw, semiconstrained four-screw, and rigid four-screw plates, respectively, were as follows: flexion: 2.6±2.0°, 1.8±1.1°, 1.8±0.8° (p=.46); extension: 2.5±2.6°, 2.1±1.3°, 1.4±1.3° (p=.45); lateral bending: 1.8±1.0°, 1.3±1.0°, 1.1±0.5° (p=.29); axial rotation: 2.9±1.9°, 1.6±0.9°, 1.5±0.7° (p=.08). Despite a tendency for the rigid two-screw plate to allow more motion than the four-screw plates, there was no significant difference among groups during any loading mode. In terms of immediate postoperative cervical stability after one-level discectomy and placement of an interbody spacer, the rigid two-screw plate performed comparably to conventional rigid four-screw and semiconstrained four-screw plates. Further research on relative fatigue endurance of the different plate types is also needed.
    No preview · Article · Mar 2011 · The spine journal: official journal of the North American Spine Society

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202 Citations
75.60 Total Impact Points

Institutions

  • 2010-2015
    • St. Joseph's Hospital and Medical Center (AZ, USA)
      Phoenix, Arizona, United States
    • NorthShore University HealthSystem
      Chicago, Illinois, United States
  • 2009-2014
    • Barrow Neurological Institute
      • Department of Neurosurgery
      Phoenix, Arizona, United States
    • Berufsgenossenschaftliche Unfallklinik Frankfurt am Main
      Frankfurt, Hesse, Germany
  • 2011
    • Howard County General Hospital
      Columbia, Maryland, United States